This invention concerns the fabrication of copper-clad laminates useful in the production of printed circuit boards and, more particularly, it concerns itself with a novel method for producing such laminates and an improved laminate product, as well as novel intermediate products.
Copper-clad laminate is one of the raw materials used in the production of printed circuit boards. Such a laminate consists of a substrate having a foil of copper firmly adhered thereto. Producers of printed circuit board (PCB) products apply the desired circuit patterns in different ways. The most common method, known as subtractive processing, involves masking the desired pattern by a photoresist or screen printed masking material on the copper-clad laminate and then removing the undesired copper cladding by etching.
Another method for producing circuit patterns requires the use of a substrate clad with ultra thin copper. Masking is applied as described above. However, the copper is exposed in the area in which the circuit pattern is desired. Electrodeposition is then performed increasing the circuit line thickness after which the masking and thin background copper are removed by etching. This approach is known as the semi-additive method.
It is desirable, of course, to produce PCB's having the maximum number of circuit lines contained thereon. The more circuit lines, and consequently, the more components, that can be fit on to a single board, the more compact and economical the product becomes. One of the limiting factors, however, in the number of circuit lines that can be applied in a given amount of space is the degree of fineness with which such lines can be produced. Another limitation is the degree of precision with which the lines themselves and the spaces between them can be defined.
Those skilled in the art realize that it is desirable in light of the foregoing objectives and for other reasons to use relatively thin foils in the production of the basic laminate product which is to be used in the production of PCB's. With the subtractive process applied to thicker foils, there is greater wastage of copper when the background foil is etched away, as described above. Also, there is necessarily a certain amount of side etching of the circuit lines themselves, reducing the amount of current carrying material and altering the surface morphology of the circuit lines. Obviously, this gives rise to a further limitation in how closely the circuit lines can be spaced from one another. Where semi-additive processing of laminates clad with thin copper foil is used, these disadvantages are clearly minimized.
Foils for copper-clad laminate have been produced for the most part by electrodeposition up to the present time. This process has many advantages, including speed of production, economy and a very fully developed technology associated with it. There are, however, certain limitations inherent in the electrodeposition process when this technology is extended to the production of ultra-thin copper foils. For one thing, it is very difficult to produce foils of less than 16 microns thickness which are free of pinholes. The pinholes appearing in thinner electrodeposited foils result in our opinion from the presence of impurities or defects at random locations on the surface of the electrode upon which deposition is occurring or as a result of entrapment of impurities inherent in the electrodeposition process. These impurities thus prevent electrodeposition at these locations creating pinholes which may close only when a certain thickness is achieved.
Furthermore, another limitation of the electrodeposition process results from the relatively large average grain size in films or foils produced thereby. With ultra-thin films or foils, particularly those in the very thinnest ranges, the average depth of the grain boundaries begins to approximate the thickness of the films themselves. Since some organic impurities will generally be collected at points in the grain boundaries there is a possible weakening of such films or foils at these points.
We have found that the foregoing disadvantages associated with the production of copper foils solely by electrodeposition can be overcome by our novel process. With our process it becomes possible to use ultra-thin foils. Also, copper-clad laminate produced in accordance with our invention offers an extremely smooth and virtually pinhole-free surface for the subsequent electrodeposition of circuit lines. Because of the uncommonly high quality and defect-free character of this surface, the overall circuit so produced by semiadditive processing will be superior in definition than has heretofore been possible. Since the foil can be thinner, the amount of etching required to remove the background copper is less, thereby tending to diminish the disadvantages associated with that process as noted above. The result of these advantages is that the laminate and the printed circuit boards produced therefrom can be made more economically, thereby lowering the cost to users. It should be noted, however, that our novel process and product have advantages over their existing counterparts even in thick foil applications. Hence, our invention is not limited strictly to the production and use of laminate with ultra-thin foils.
Briefly described, our method involves the vapor deposition of a film of metal, preferably copper, on a carrier such as aluminum sheet in such a way as to produce a relatively weak adherent bond therewith. In our method, we prevent strong adhesion between the film and the aluminum carrier sheet by coating the sheet with silicon dioxide or another substance suitable for the purpose. The thickness of that coating may, for example, be as thin as 200 to 600 Angstroms and may be much thicker up to the point that the physical integrity of the coating is not sufficient for the coating to withstand the stripping or other foil separation step described herein. After the production of the film on the carrier sheet, the exposed surface of the vapor deposited film is given an electrolytic treatment to develop the bonding layer to complete the foil for its ultimate lamination with an appropriate substrate such as glass epoxy. Next, the exposed surface of the completed foil is pressed against the intended substrate at appropriately elevated temperatures to bring about the intended lamination by embedding the bonding layer in the substrate. Once the lamination is completed, the carrier sheet can be left in place to serve as a protective covering. At a later time the carrier sheet may be stripped away taking the coating of release agent with it and leaving the copper foil with its vapor deposited surface exposed. Since that surface was produced by vapor deposition, it will have an average grain size on the order of 500 Angstroms or less which is about a factor of 20 smaller than the grain size associated with electrolytically deposited copper.
In a similar method the coating of release agent and copper foil are applied to a stiff, flat, smooth, metal surface, e.g., a stainless steel press pan.
Likewise briefly described, a metal clad laminate product of this invention comprises a substrate and a metal foil adhering thereto, the foil including an electrolytically-deposited bonding layer embedded into the substrate and further including a vapor deposited film overlying and integral with the bonding layer and providing an exposed surface of relatively small grain size for the laminate.
Similarly another product of this invention comprises a carrier sheet coated with a release agent which preferably takes the form of a layer of vapor deposited silica.
Still another of these new products, broadly and generally defined, is a copper-clad laminate of a carrier sheet coated with a release agent and a copper film covering and adhering to the release agent, the copper film having an average grain size of about 500 Angstroms in its surface in contact with the release agent.